The invention concerns a nuclear magnetic resonance (NMR) multi-slice imaging method for recording N slices (N>1) from a measuring object, wherein for complete image reconstruction, an acquisition is used in m (m>1) repetition steps with different spatial encoding each, with a magnetization preparation in the form of a saturation pulse which is selective with respect to chemical shift and which is applied P times (1≦P<N) in each repetition step such that through each of these chemical shift selective saturation pulses the spins to be saturated of one of the N slices in the measuring object are optimally saturated and consequently the signals of N-P slices are saturated only sub-optimally.
A method of this type is e.g. known from the publication by Rosen B R, Wedeen V J, Brady T J. “Selective saturation NMR imaging”, J Comput Assist Tomogr; 8:813-818; 1984 mentioned in the appended reference list under [2].
Imaging nuclear magnetic resonance tomography obtains two-dimensional slice images from an object to be investigated. A volume is imaged by recording sets of parallel slices which extend in the spatial direction orthogonal to these slices.
Recording of an image requires in general m partial steps, wherein n complex data points are acquired in each case. The image reconstruction from the m×n data points is carried out by means of two-dimensional Fourier transformation (2DFT). Between two partial steps, the so-called phase encoding steps (PE steps), the magnetic field is varied in each case through temporally varying magnetic field gradients. Generally, a slice is excited several times to acquire several phase encoding steps. The time between two excitations, the so-called repetition time TR, is in the order of magnitude of NMR relaxation times and determines the image contrast.
Frequency-selective (narrow-band) RF pulses, which are irradiated at a constant magnetic field gradient, permit selective excitation of slices of a certain thickness. One class of methods is called multi-slice imaging wherein a number of N slices is successively excited and read out within a repetition time TR. All slices are excited in the same order in each repetition step. Spatial encoding is different in the individual repetition steps [1]. The time interval between two subsequent excitations of the same slice equals the repetition time TR=N*TA for all slices, when TA is the time required for acquiring a phase encoding step. All N slices are recorded in almost the same acquisition time, which is required for recording one slice.
The Larmor frequency of the protons bound in the fat molecules varies by 3.4 ppm from the free protons or those bound in water. This so-called chemical shift makes it principally possible to produce separate water or fat images, which contain only one of the two spectral components each. A commonly used method of obtaining water images is the so-called fat saturation by means of chemical shift selective RF pulses.
Magnetization of the fat molecules is deflected into the transverse plane and spoiled by gradient pulses directly before spatial (slice) selective excitation by means of a chemical shift selective pulse. After this preparation, which consists of application of a fat saturation pulse and spoiler gradients, the slice excitation acting on both spectral components and the actual data acquisition of the remaining water signal are carried out [2].
For most sequences, fat saturation preparation is placed directly before each spin echo or gradient echo acquisition. Other techniques such as fat saturated multiple spin echo sequences read out larger parts of the k space after only one preparation [3].
A faster variant can also be used for multi slice recordings. Fat saturation preparation before each acquisition of a phase encoding step of each slice is dispensed with. At the start of each repetition only one preparation is applied. Subsequently, all slices are excited one after the other before applying the next preparation in a following repetition time. A set of water or fat images can be recorded in approximately the same time as the corresponding combined images [2,4,5].
The time interval between preparation and excitation of a certain slice is different for the different slices. The magnetization deflected by fat saturation preparation experiences different longitudinal orientation for different times due to T1 relaxation. Each slice is acquired in a different preparation state. Fat suppression can be optimized only for one slice by exciting this slice at the point in time during the repetition time when the longitudinal magnetization of the fat protons is zero. The other slices show reduced fat suppression corresponding to T1 relaxation and the order of slice excitation.
In contrast thereto, it is the object of the present invention to improve a method of the type described in the beginning such that the above-discussed disadvantages can be avoided. The invention shall present in particular a new method aiming for fat or water suppression, which is uniform for all slices.